What is it about speed that upsets the shipping industry?

You won’t find many – if any – who disagree that the higher the speed of a ship, the higher the level of greenhouse gas emissions. So why would I argue that, while imposing speed limits may have a popular ring to it, it is not the answer as the industry seeks to cut emissions? After all, our mission is to do exactly that: cut emissions.

The important question is not whether a speed limit should be introduced to cut emissions, but how it would be checked. Before vouching for an argument, one must look at whether it can be done.

What looks good on paper does not always work in practice. Enforceability is a very important aspect of any meaningful regulation. Can authorities check for compliance? Is the regulation meaningful if it can’t be checked?

Only one option would work in practice

I can think of four possible options, but only one works when it comes to implementing a solution that can be measured and checked accurately, and that correlates to emissions:

• A limit for speed through water. This cannot be checked accurately, but correlates closely to emissions.
• A limit for speed over ground. This can be checked accurately, but there is a much lower correlation to emissions.
• A limit for average speed (over ground or through water). It may be possible to check this, but there is also a much lower correlation to emissions.
• Limits for propulsion power. This is the only option that can be checked accurately and has a close correlation to emissions.

To understand why this last option is the best solution, we must get technical.

Most people have an intuitive understanding of what speed means, and that is usually derived from personal observations when we move on the ground. If you move 7km in one hour, your average speed is 7km per hour. Short and simple.

What is less simple is recognising that, when we move on the ground, there is no slip. The wheels on our car turn one revolution, and the car has moved exactly the distance equal to the circumference of the wheel.

It is different at sea – and, for that matter, when we talk about movement in the air.

Measuring a ship’s speed through water is not accurate

When a ship moves, it moves through water. It also moves over ground – and so does the water in which the ship sails. Observing a ship’s speed over ground is really an observation of the ship’s speed through water plus or minus the current at any given time.

The problem is, we do not know the current at any given time, and measuring a ship’s speed through water directly is not very accurate, either.

When a ship is new, and the shipyard measures its speed/power curve, it does so by conducting a very precisely measured double run between two fixed positions: there and back. This eliminates the influence of current and results in an average speed over ground that accurately reflects the ship’s speed through water.

When a ship is in operation, it is impractical to measure in this way. The shipping company still wants to keep an eye on the ship’s performance, and this is often done by observing propeller revolutions and factoring in an average slip percentage for the ship. Slip is the relationship between the observed movement of the ship when the propeller turns one revolution in water and the propeller’s theoretical movement of the ship had the water been a solid material.

Some ships have doppler speed logs, which use advanced techniques to measure the speed of the water column below the ship relative to the ship itself. Still, the water flow is not laminar close to the ship hull and such devices need frequent adjustment to produce accurate results.
In summary, measuring a ship’s speed through water is not a precise exercise.

Low correlation between emissions and speed over ground

If you measure a ship’s speed over ground and try to correlate this to the emissions or the power of the engine, you would get a very large scatter. This is because the current changes by time and location – and it changes significantly. The difference between favourable currents with a ship and unfavourable currents against a ship may be as large as 50% of the speed observed over ground.

So there is a low correlation between a ship’s emissions and its speed over ground.

Cutting average speed – where is the emissions connection?

When we look at average speed – the third option, above – we need to bear in mind how speed through water and power correlates for a ship to understand if this could work. Remember the rule of thumb: power = constant x speed3.

A ship travelling between two locations, sailing at constant speed, emits 100% CO2. A ship travelling between the same two locations – sailing the first half distance at 50% higher than the average speed, and the second half distance at half of that higher speed – would emit 141% CO2. These two scenarios give the same average speed for the ship.

So there is no correlation between average speed through water and emissions. Averaging speed over ground just make things even more arbitrary.

Emissions are driven by power of the engine

All this may seem unimportant, but emissions are driven by the power of the engine that turns the ship’s propeller. There is very good correlation between emissions and power of the engine.

There is also a reasonably good correlation between a ship’s emissions and its speed through water, as mentioned in the first option. We must keep in mind, however, that two ships with the same cargo-carrying capacity – but different efficiency – would have different emissions. So the correlation between many ships travelling at the same speed and their emissions is not good.

We can continue to talk about speed limits for ships, but if we forget to ask ourselves the fundamental question of whether it helps cut emissions, we are heading in the wrong direction on our way to the industry’s 2050 greenhouse gas emission targets.

Limiting emissions via a ship’s power to the propeller not only gets us in the right direction, it encourages innovation around more efficient ships – and helps us assure a level competitive playing field on our way there.
Source: BIMCO Bulletin Magazine – June 2019 issue (http://portfolio.cpl.co.uk/BIMCO/201906/cover/)